¹Lilly Research Laboratories, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN, USA

Correspondence: Dr K Rasmussen, Lilly Research Laboratories, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA. Tel: +1 317 277 8835, Fax: +1 317 276 5546, E-mail: Rasmussen_kurt@lilly.com

Received 28 April 2006; Revised 11 September 2006; Accepted 13 September 2006; Published online 25 October 2006.

Abstract

Antipsychotic drugs alter the activity of dopamine neurons in the ventral tegmental area (A10) and substantia nigra pars compacta (A9). As there is a dense projection of orexin neurons from the lateral hypothalamus to A10 dopaminergic neurons, and some antipsychotics have been shown to increase the expression of c-fos in orexin-containing cells in the hypothalamus, we hypothesized that stimulation of orexin receptors plays a role in the effects of antipsychotics on the activity of A9 and A10 dopamine cells. Single-unit recordings in anesthetized rats demonstrated the central effects of the selective orexin-1 receptor antagonist SB-334867 (2 mg/kg, intravenous), as it reversed the excitatory effects of orexin-A administration (6 g, intracerebroventricular) on the activity of locus coeruleus (LC) cells. Recordings from midbrain dopamine neurons showed that acute administration of SB-334867 alone did not alter the number of spontaneously active A9 or A10 cells, but did reverse: (1) the increase in the number of spontaneously active A9 and/or A10 dopamine cells caused by the acute administration of haloperidol (1 mg/kg, subcutaneous) or olanzapine (10 mg/kg, s.c.) and (2) the decrease in the number of spontaneously active A9 and/or A10 dopamine cells caused by the chronic administration of haloperidol (1 mg/kg/day 21 days, s.c.) or olanzapine (10 mg/kg/day 21 days, s.c.). However, SB-334867 did not block a different electrophysiological effect of olanzapine, as it did not block the olanzapine-induced activation of LC cells. These results indicate that activation of orexin-1 receptors plays an important role on the effects of antipsychotic drugs on dopamine neuronal activity and may play an important role in the clinical effects of antipsychotic drugs.